In a downhole oil well logging tool, electronic circuitry is routinely exposed to very high ambient temperatures. The temperature in an oil well increases with depth. Regretably, most electronic components used in the electronics placed in downhole tools are degraded with increasing temperature, and the increase in temperature markedly limits the performance of the components. Heating of electronic components in a downhole tool never occurs uniformly among the many components. Inevitably, one circuit component will run relatively cool while the current flow through another component will heat that component to fairly high temperatures. The performance of the circuit, considered as a whole, is severely degraded by the heating of a single component notwithstanding all the other components are operated several degrees cooler. This inevitably poses a problem in cooling the hottest component.
There are space and access limitations within a downhole wireline oil well tool. Such tools are typically constructed with elongate cylindrical bodies, and the electronic components are normally mounted therein on printed circuit boards (PC boards hereafter). They are normally rectangular boards, typically mounted in a vertical position within the tool, sometimes arranged with two or three parallel layers between transverse wafers or bulkheads. Often, a downhole tool will be divided into various and sundry compartments by the wafers or bulkheads. This severely limits air circulation around components. While air can provide modest cooling, convection cooling is usually very limited by physical constraints. Radiation cooling is likewise very limited in effectiveness. Attempts have been made in the past to cool downhole equipment. For instance, Hallenburg has disclosed in U.S. Pat. Nos. 3,435,629 and also 3,488,970 approaches to this problem. The latter patent has a heat transfer system including the Peltier effect with a water sump. The earlier Hallenburg reference describes a pump with two separate chambers for the coolant liquid.
The most recent patent of Lamers, et al, U.S. Pat. No. 4,248,298 shows a cooling system using a surface reservoir. The full flow path extends from surface to downhole tool, and back to the surface. This is practically impossible of construction because it would require conduits in a logging cable up to perhaps 25,000 feet in length. The recent Rothwell patent, U.S. Pat. No. 4,224,805, shows a surface refrigeration system with certain downhole cooling as set forth in that disclosure.
By contrast, this apparatus locates a sump immediately below PC boards to be cooled. In actuality, the PC boards are not cooled; really, the electronic components which are mounted on them are cooled. It is convenient, however, in speaking of a PC board to refer to an installed board with the components on it, the components typically comprising integrated circuits, transistors, capacitors, resistors and the like.
This apparatus cools a PC board by positioning a sump for a liquid refrigerant therebelow. The refrigerant is brought, as a liquid, to the PC board. It is sprayed, splashed or dripped onto the PC board for cooling. Cooling is accomplished with a change in phase. The vapor liberated by absorbing heat is collected in the near area and is subsequently condensed. In one embodiment, it is condensed by a surrounding metallic shell isolating the chamber above the sump, and the condensing liquid runs down the interior surface of the shell to collect in the sump. In another embodiment, a heat sink capable of absorbing heat cools the refrigerant vapors to condense on the outer surface. In both cases, a refrigerant flow cycle is incorporated, beginning with liquid in the sump, cooling of electronic components by applying liquid to them on the PC boards, vaporization of the refrigerant applied to the components, and condensation of the vapor to form a liquid which is again returned toward the sump.